218 Bacteria in Relation to Country Life 



pheric nitrogen when developing within the nodules. 

 The term virulence has been used to designate this 

 variable power of the bacteria. Organisms of a high 

 degree of virulence readily penetrate the legume roots, 

 and secure large quantities of nitrogen from the air. 



Organisms of a low de- 

 gree of virulence are 

 feeble in this respect. 



Under favorable soil 

 conditions the virulence 

 of the bacteria may be 

 increased; under un- 

 fayorable soil condi- 

 tions their virulence may 

 be greatly diminished. 

 Hence, soils well pro- 

 vided with lime and 

 humus, soils in good 

 tilth, well aerated and 

 supplied with an abun- 

 dance of moisture, will 



the fixation of 



Fig. 36. Root tubercles of unhealthy al- 



falfa plants. The tubercles are large large quantities of at- 

 and few in number. v , 



mosphenc nitrogen. On 



the other hand, ill-drained soils, or those deficient in 

 humus, will tend to diminish the virulence of the 

 nodule-bacteria, to the injury of the legume crops that 

 may be grown upon them. 



Balance. The conception of resisting power of plants 

 on the one hand and that of the virulence of bacteria 

 on the other, introduce a third conception of a balance 



